The pyridine alkaloids of tobacco (Nicotiana tabacum L.) are among the most studied group of plant secondary compounds. Nicotine constitutes greater than 90% of the total alkaloid pool in most tobacco genotypes and is primarily responsible for the pharmacological response experienced by users of tobacco products. In decreasing order of relative abundance, the remaining major alkaloids in tobacco include anatabine, nornicotine, and anabasine. Alkaloid levels in tobacco are influenced by environmental conditions, interactions with plant pests, and plant genetics.
Although nicotine is the primary compound that gives the users of tobacco products the pharmacological effect they seek, there are several circumstances where it would be desirable to develop products using tobacco plants that produce and accumulate very low levels of nicotine. For example, some studies have shown that the use of low-nicotine cigarettes as a component in smoking cessation strategies can help smokers who are trying to quit (Hatsukami et al., 2010a; Donny et al., 2014). Other reports have demonstrated that by lowering the nicotine levels below a critical threshold in tobacco products, they can no longer initiate or maintain an addiction response (Benowitz and Henningfield, 1994; Benowitz et al., 2007). Studies such as these may ultimately influence regulatory agencies, such as the U.S. Food and Drug Administration, who have been given the authority to determine what acceptable levels of various tobacco constituents (including nicotine) will be allowable in cigarettes and other tobacco products.
Tobacco alkaloid levels are also of interest because of their role in the production of tobacco specific nitrosamines (TSNAs), a potent group of recognized carcinogens (Hecht, 1998, 2003; Hecht and Hoffman, 1989). The most important TSNAs are N-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), which are derived through nitrosation reactions with nornicotine and an oxidative derivative of nicotine (such as pseudooxynicotine), respectively, during the curing, storage, and consumption of tobacco. Because tobacco alkaloids serve as precursors toward TSNA formation, low alkaloid tobacco plants have also been shown to produce reduced amounts of TSNAs within the cured leaf (Xie et al., 2004). Further, while modifications in the curing environment have led to substantial TSNA reductions in flue-cured tobacco varieties, this has not been the case in the air-cured burley tobacco types.
This invention addresses the need for compositions and methods that modulate the nicotine biosynthesis pathway in plants.